Sql server® 2012 t-sql fundamentals

default Constraints

A default constraint is associated with a particular attribute. It is an expression that is used as the de-

fault value when an explicit value is not specified for the attribute when you insert a row. For example, 

the following code defines a default constraint for the orderts attribute (representing the order’s time 

stamp):

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CHAPTER 1

 

Background to T-SQL Querying and Programming

 

 25

ALTER TABLE dbo.Orders 

  ADD CONSTRAINT DFT_Orders_orderts 

  DEFAULT(SYSDATETIME()) FOR orderts;

The default expression invokes the SYSDATETIME function, which returns the current date and 

time value. After this default expression is defined, whenever you insert a row in the Orders table 

and do not explicitly specify a value in the orderts attribute, SQL Server will set the attribute value 

to SYSDATETIME.

When you’re done, run the following code for cleanup.

DROP TABLE dbo.Orders, dbo.Employees;

Conclusion

This chapter provided a brief background to T-SQL querying and programming. It presented a theo-

retical background, explaining the strong foundations that T-SQL is based on. It gave an overview of 

the SQL Server architecture and concluded with sections that demonstrated how to use T-SQL to cre-

ate tables and define data integrity. I hope that by now you see that there’s something special about 

SQL, and that it’s not just a language that can be learned as an afterthought. This chapter equipped 

you with fundamental concepts—the actual journey is just about to begin.

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